If you feel nervous about your viva then you’re reacting to the importance of the event. In anticipation you feel that you want everything to go well and that’s the nerves you feel.
If you feel excited about your viva then you’re also reacting to the importance of the event. You feel more certainty than a nervous person perhaps, and can’t wait for the viva to go well.
Feeling nervous isn’t wrong. Feeling excited isn’t wrong. Neither are end states. They change and can be made to change. Feelings can help to prompt your action.
Feeling nervous for the viva? What do you need to do to help (as nerves aren’t always comfortable)?
Feeling excited for the viva? What do you need to do to maintain your good feelings (as they might still need encouragement)?
How do you feel? So what do you need to do?